Add tool/package_linux.sh: release tar.gz + Discord upload card

One command: flutter build linux --release, then bundles into
dist/watch-together-<version>-linux-x64.tar.gz with a single
top-level folder (executable, lib/, data/). Uploads via
DISCORD_WEBHOOK_URL env (never committed) as a Components V2 card:
version header, short git commit, stat codeblock, tarball as File
component. --noupload skips, --skip-build reuses the last bundle.
Verified: 0.1.0 builds, exec bit preserved, payload asserted
against a stub (flag 32768, container+file shape, attachment ref).

tool/package_linux.sh

@@ -0,0 +1,121 @@
+#!/usr/bin/env bash
+# Package the Linux release bundle into a versioned tar.gz and optionally
+# upload it to Discord.
+# Usage: tool/package_linux.sh [--skip-build] [--noupload|--no-upload]
+# Output: dist/watch-together-<version>-linux-x64.tar.gz containing one
+# top-level folder with the executable, lib/ and data/ as built.
+# Upload: set DISCORD_WEBHOOK_URL in the environment (never commit it).
+# The tarball must stay under Discord's upload limit (25 MB free tier).
+# The upload posts a Components V2 card (flag 1<<15): version header,
+# short git commit, stat codeblock, and the tarball as a File component.
+# Needs: curl, git, python3 (JSON payload building).
+set -euo pipefail
+
+SKIP_BUILD=0
+UPLOAD=1
+for arg in "$@"; do
+  case "$arg" in
+    --skip-build) SKIP_BUILD=1 ;;
+    --noupload|--no-upload) UPLOAD=0 ;;
+    *) echo "usage: $0 [--skip-build] [--noupload|--no-upload]" >&2; exit 1 ;;
+  esac
+done
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+APP="$ROOT/apps/desktop"
+NAME="watch-together"
+ARCH="x64"
+
+VERSION="$(sed -n 's/^version: \([0-9.]*\).*/\1/p' "$APP/pubspec.yaml")"
+if [[ -z "$VERSION" ]]; then echo "error: version not found in $APP/pubspec.yaml" >&2; exit 1; fi
+
+if [[ "$SKIP_BUILD" != "1" ]]; then
+  (cd "$APP" && flutter build linux --release)
+fi
+
+BUNDLE="$APP/build/linux/$ARCH/release/bundle"
+[[ -d "$BUNDLE" ]] || { echo "error: bundle missing (build first): $BUNDLE" >&2; exit 1; }
+
+STAGE="$APP/build/package-stage"
+DIST="$ROOT/dist"
+FOLDER="$NAME-$VERSION"
+TARBALL="$DIST/$FOLDER-linux-$ARCH.tar.gz"
+rm -rf "$STAGE"
+mkdir -p "$STAGE/$FOLDER" "$DIST"
+cp -r "$BUNDLE/." "$STAGE/$FOLDER/"
+tar -czf "$TARBALL" -C "$STAGE" "$FOLDER"
+rm -rf "$STAGE"
+ls -la "$TARBALL"
+
+if [[ "$UPLOAD" == "1" ]]; then
+  if [[ -z "${DISCORD_WEBHOOK_URL:-}" ]]; then
+    echo "warning: DISCORD_WEBHOOK_URL not set — packaged but not uploaded." >&2
+  else
+    command -v curl >/dev/null || { echo "error: curl not found" >&2; exit 1; }
+    command -v python3 >/dev/null || { echo "error: python3 not found" >&2; exit 1; }
+    PAYLOAD="$(mktemp)"
+    trap 'rm -f "$PAYLOAD"' EXIT
+    export PKG_ROOT="$ROOT" PKG_PAYLOAD="$PAYLOAD" PKG_TARBALL="$TARBALL" \
+      PKG_FOLDER="$FOLDER" PKG_VERSION="$VERSION" PKG_ARCH="$ARCH"
+    python3 <<'EOF'
+import json, os, subprocess
+
+root = os.environ['PKG_ROOT']
+out = os.environ['PKG_PAYLOAD']
+tarball = os.environ['PKG_TARBALL']
+fname = os.path.basename(tarball)
+folder = os.environ['PKG_FOLDER']
+ver = os.environ['PKG_VERSION']
+arch = os.environ['PKG_ARCH']
+size_mb = os.path.getsize(tarball) / 1048576
+
+def git(*args):
+    try:
+        p = subprocess.run(['git', '-C', root, *args],
+                           capture_output=True, text=True, timeout=10)
+        return p.stdout.strip()
+    except Exception:
+        return ''
+
+commit = git('log', '-1', '--format=%h %s') or 'uncommitted build'
+stat = git('show', '--stat', '--oneline', '--format=', 'HEAD')
+stat_lines = [l for l in stat.splitlines() if l.strip()][:14]
+rest = len([l for l in stat.splitlines() if l.strip()]) - len(stat_lines)
+if rest > 0:
+    stat_lines.append(f'... +{rest} more files')
+block = '```\n' + '\n'.join(stat_lines) + '\n```'
+
+payload = {
+    'flags': 1 << 15,
+    'attachments': [{
+        'id': 0,
+        'filename': fname,
+        'description': f'{folder} linux {arch} release build',
+    }],
+    'components': [{
+        'type': 17,
+        'accent_color': 5793266,
+        'components': [
+            {'type': 10, 'content':
+             f'# 📦 watch-together `{ver}`\n'
+             f'-# linux · {arch} · release · {size_mb:.1f} MB'},
+            {'type': 10, 'content': f'`{commit[:140]}`'},
+            {'type': 10, 'content': block[:1500]},
+            {'type': 14, 'divider': True, 'spacing': 1},
+            {'type': 13, 'file': {'url': f'attachment://{fname}'}},
+        ],
+    }],
+}
+with open(out, 'w') as f:
+    json.dump(payload, f)
+EOF
+    # Plain (non-application) webhooks ignore `components` unless the
+    # with_components query param is set — without it Discord 500s.
+    SEP='?'; [[ "$DISCORD_WEBHOOK_URL" == *\?* ]] && SEP='&'
+    curl -fsS \
+      -F "payload_json=<$PAYLOAD;type=application/json" \
+      -F "files[0]=@$TARBALL;type=application/gzip" \
+      "${DISCORD_WEBHOOK_URL}${SEP}with_components=true" > /dev/null
+    echo "uploaded $FOLDER-linux-$ARCH.tar.gz"
+  fi
+fi